BRITNEY Spears has set alarm bells ringing after storming off the set of X Factor USA – on her very first day as a judge.

The troubled 30-year-old walked away, leaving the final four contestants to perform for remaining judges Simon Cowell, Demi Lovato, L.A. Reid and her empty chair.

Onlookers at the auditions in Austin, Texas, said the audience was stunned at the singer’s behaviour, branding her unprofessional.

Britney’s inclusion on the judging panel – in a deal worth just under £10million for the series – raised eyebrows, with fans fearing she would struggle to cope with the pressure after a string of public breakdowns.

As a result, her personal and professional decisions are overseen by her father Jamie and her fiance Jason Trawick.

Dressed in a bright pink minidress, she quit the stage, followed by Jason, when a hopeful butchered her 2011 single Hold It Against Me.

One audience member at Thursday’s auditions said: “Britney just stood up and disappeared. Nobody knew what was going on and there were anxious faces around the production team.

“She was gone for a few minutes and everyone expected her to come back to her seat, but then the next singer was on stage and the show just carried on.

“It doesn’t bode well for the rest of the series. She’s only been there a day and she’s already causing a scene. It was very unprofessional.”

But the Toxic singer yesterday defended the walkout, insisting she was just taking a “little break”.

She wrote on Twitter: “Was just taking a little break, people. I am having the best time!”

Her appearance, five years after shaving her head during a 2007 breakdown, was a gamble for show supremo Cowell.

He is busy revamping The X Factor USA after the first series attracted only half the number of viewers he predicted.

Nicole Scherzinger and Paula Abdul were axed from the judging panel, while host Steve Jones was also dropped.